The present invention relates to a retiming arrangement for use in an SDH (Synchronous Digital Hierarchy) data transmission system. In particular the invention is concerned with the provision of an arrangement by which third parties wishing to use an SDH link which is controlled by another operator may carry their own timing and other information across that network.
Third party timing is defined when the timing to be carried in a primary rate signal originates from a source which is not synchronous with the source used to time the SDH network. Third party timing is therefore plesiochronous to the bearer clock.
In SDH, primary rate signals are mapped into high bit rate Synchronous Transport Modules (STM-N), at the insertion point of the network using virtual containers (VC) and tributary units (TU), the size and type of which depends on the data rate of the signal being carried. Many of these TU's are multiplexed together into a single STM-N. At the extraction point the STM-N is demultiplexed and demapped back into primary rate signals. However the signals suffer from phase and frequency distortions which effect the quality of the signal when used for transporting timing information. The key component in SDH systems used to reduce these distortions is the so-called desynchronizer or retiming arrangement.